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The Time-Transgressive Termination of the African Humid Period

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The Time-Transgressive Termination of the African Humid Period SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO2329 The time-transgressive termination of the African Humid TheThe time-transgressive time-transgressive termination termination of the African of Humid the African Period Humid Period Period Timothy M. Shanahan, Nicholas P. McKay, Konrad A. Hughen, Jonathan T. Overpeck, TimothyBette Otto-Bliesner, M. Shanahan, Clifford Nicholas W. Heil,P. McKay, John King,Konrad Christopher A. Hughen, A. Jonathan Scholz, John T. Overpeck, Peck Bette Otto-Bliesner, Clifford W. Heil, John King, Christopher A. Scholz, John Peck Supplementary Information Supplementary1. Supplementary MethodsInformation 1.1.1. Supplementary Study Area Methods Lake Bosumtwi is a small (~8 km diameter) but deep (~75 meter) stratified lake 1.1. Study Area occupying a meteorite impact crater in the tropical forest zone of southern Ghana (Fig. Lake Bosumtwi is a small (~8 km diameter) but deep (~75 meter) stratified lake S1, S2). The lake is internally draining and isolated from the regional groundwater occupying a meteorite impact crater in the tropical forest zone of southern Ghana (Fig. system, making it exceptionally sensitive to changes in the precipitation-evaporation S1, S2). The lake is internally draining and isolated from the regional groundwater balance 1-3. Sheltering of the lake by the surrounding crater walls limits deep mixing of system, making it exceptionally sensitive to changes in the precipitation-evaporation the water column, resulting in permanently anoxic bottom waters and the preservation of balance 1-3. Sheltering of the lake by the surrounding crater walls limits deep mixing of fine (mm-scale) laminations, which have been previously demonstrated to be annual in the water column, resulting in permanently anoxic bottom waters and the preservation of nature 4. Most of the surrounding catchment is forested, except the flat-lying areas, fine (mm-scale) laminations, which have been previously demonstrated to be annual in which have been converted to agriculture (e.g., maize, plantain, cassava) over the last few nature 4. Most of the surrounding catchment is forested, except the flat-lying areas, decades 5. which have been converted to agriculture (e.g., maize, plantain, cassava) over the last few In 1999 and 2004, a series of freeze cores, piston cores and drill cores were decades 5. collected from the lake depocenter (Fig. S1)6. The freeze cores capture the sediment In 1999 and 2004, a series of freeze cores, piston cores and drill cores were water interface intact and were visually correlated to the piston cores using marker collected from the lake depocenter (Fig. S1)6. The freeze cores capture the sediment laminations that could be traced in sediment cores across the central basin 4. The piston water interface intact and were visually correlated to the piston cores using marker cores are finely (mm-scale) and continuously laminated over their entire length, with the laminations that could be traced in sediment cores across the central basin 4. The piston exception of the interval between 2.1 and 3.9 meters, which is organic-rich and contains cores are finely (mm-scale) and continuously laminated over their entire length, with the abundant well-preserved remains of the cyanobacteria Anabaena indicative of eutrophic exception of the interval between 2.1 and 3.9 meters, which is organic-rich and contains conditions (Unit S1) 4. Drill cores collected during the Intercontinental Drilling Program abundant well-preserved remains of the cyanobacteria Anabaena indicative of eutrophic (ICDP) sediment coring expedition to the lake in 2004 provide a nearly continuous 290- conditions (Unit S1) 4. Drill cores collected during the Intercontinental Drilling Program (ICDP) sediment coring expedition to the lake in 2004 provide a nearly continuous 290- NATURE GEOSCIENCE | www.nature.com/naturegeoscience 1 1 © 2015 Macmillan Publishers1 Limited. All rights reserved meter long sediment record, which spans the last ca. 1.07 Ma 6. The present study focuses on the uppermost 10.6 meters of this sediment sequence preserved in the piston cores, which covers the last 25 kyr. Located in southern Ghana, the climate of Lake Bosumtwi is heavily influenced by the West African monsoon, which draws moisture onshore from the Gulf of Guinea during the months of April-October as the monsoon trough migrates northward with the sun (Fig. S2). In winter the winds reverse, and the climate of southern Ghana is influenced by hot, dry northeasterly monsoon winds, which inhibit precipitation over much of North Africa. Because of its near equatorial (6°N) location and its proximity to the coast, the rainy season is longer and precipitation totals are high near the coast and around Lake Bosumtwi (e.g., 1260 mm yr-1) and decrease with increasing latitude 3. 1.2 Age depth modeling A radiocarbon chronology for the sediment record is based on Bayesian age-depth modeling of 107 radiocarbon ages on plant macrofossils and bulk organic matter covering the upper 21.4 meters (Fig. S3) 7. Modeling was performed with the R software package BACON 8 and using the IntCal09 radiocarbon calibration curve 9. Radiocarbon dates from overlapping cores were correlated using matches from marker laminae and magnetic susceptibility profiles and placed on a standardized depth scale (relative meter composite depth scale – RMCD)7. For modeling purposes, BACON uses the prior assumption that ages are in stratigraphic order and that sedimentation rates fall within a designated range and follow a long-tailed distribution. The program models sedimentation rates using a simple gamma autoregressive process and generates posterior age distributions using a self-adjusting Markov Chain Monte Carlo algorithm 10. The result is a population of possible age-depth models, which reflect the probability distributions associated with the analytical radiocarbon age uncertainties as well as the uncertainties in the calibrated ages. 1.3 Reconstruction of paleolake highstands The paleolake level history was reconstructed using a combination of radiocarbon dated highstand benches identified above the modern lake level, lowstand terraces 2 © 2015 Macmillan Publishers Limited. All rights reserved identified in seismic data and sediment cores, and lacustrine muds deposited above the modern lake and preserved on the crater walls (Fig. S4, main text, Fig 2b.; for a more detailed description see 1,11). Radiocarbon dates on roots from a submerged paleoterrace indicate that the lake was at least 60 meters lower at 16.3 ka 1,12. However, laminated lacustrine silts 20 meters above the modern lake level indicate that the lake was already substantially deeper than today by 14.5 ka. This recovery was interrupted by a lake level regression between 12.9-11.2 ka, followed by a return to deep lake conditions in the early Holocene. The early Holocene highstand deposited extensive lacustrine silts on the walls of the crater, up to 110 meters above the modern lake, until at least 5.7±0.4 ka 1, though significant undated lacustrine silt accumulations above this date indicate that the high lake stand persisted much later than this. A wave-cut notch in the crater rim indicates that the positive water balance during the early Holocene resulted in the lake overflowing the crater rim 4. Although the lake level record does not completely constrain the end of the Holocene lake level highstand, a well-dated terrace 15 meters above the modern lake level suggest that it had mostly receded by ca. 2.8-3.5 ka. Submerged terraces, identified in seismic data and confirmed in proxy data suggest that the lake shrank by more than 25 meters below modern lake level at least twice in the last 2 millennia, the most recent of which occurring coincident with the Little Ice Age (1500-1800 AD) 1,13. Constraints on the timing of the maximum lake level come from radiocarbon dating of a maximum highstand terrace ca. 110 meters above the modern lake surface and cosmogenic surface exposure dating of wavecut bedrock in the overflow spillway at the same elevation. Radiocarbon dates on charcoal from the base of the highstand terrace yield an age of 8.6-9.0 ka, which we interpret as indicating the onset of overflowing conditions 1. Cosmogenic radiocarbon surface exposure ages on eroded bedrock from the overflow provide an estimate of when water ceased overflowing from the crater. Earlier exposure ages estimates were similar to that obtained from the highstand terrace (9.5±1.5 ka) 1. However, a revised age estimate using updated cosmogenic 14C production rate estimates and scaling procedures (assuming a spallation production rate of 13.6 atom/gram/yr at sea level and high latitude 14; and computed with the Cronus calculator 15), yielded significantly younger ages (5.9±0.9, 6.2±0.9 ka). Together these data suggest 3 © 2015 Macmillan Publishers Limited. All rights reserved that the lake overflowed for a significant portion of the early Holocene (i.e., between ca. 9.0 and 6.0 ka), consistent with the δD record. 1.4 Stable isotope analysis of leaf waxes 1.4.1 n-alkane extraction and separation 1-5 gram sediment samples were freeze dried and solvent extracted using either a Dionex Accelerated Solvent Extraction System or a CEM MARS X Microwave Extraction System with dichloromethane:methanol (9:1; v/v) to obtain a total lipid extract (TLE). The TLE was partitioned into acid and neutral fractions by aminopropyl flash chromatography using 9:1 DCM:MeOH and 2% formic acid in DCM. The neutral fraction concentrated under N2 and separated into polar and apolar fractions over silica gel with hexane and MeOH. Either urea adduction or molecular sieve was used to remove branched compounds and when needed, Ag+ chromatography was used to isolate the alkane and alkene fractions prior to stable isotope analysis. 1.4.2 Hydrogen isotope analysis of n-alkanes Hydrogen isotope analysis of individual n-alkanes was performed using an Agilent 6890 gas chromatograph operated in splitless mode and equipped with a DB-5 ms column (30m x 0.25µm x 0.25mm), coupled to a Delta V isotope ratio mass + spectrometer via a pyrolysis interface operated at 1430°C.
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